On the use of high-Tc superconductors for Perot-Fabry mirrors coatings

In a very rough experiment using two MgO plates, coated with a 200 Å thick YBaCuO film to increase their reflectivity, and facing each other their YBaCuO films to make a Perot-Fabry (PF) interferometer, we have observed a 43% maximum transmission and a finesse around 33 at v=29 cm^–1, in good accordance with a new phenomenological model for YBaCuO. Much higher performances can be expected by optimizing the YBaCuO film thickness.     

Zero-field current-voltage characteristics in high-Tc superconductors

Zero-field current-voltage (I–V) characteristics of various high-temperature superconductor samples are analyzed in the context of the current-temperature (I−T) phase diagram. After establishing the validity and relevance of the phase diagram to these materials, the anisotropy factor is extracted from the slope ofI _c ¹ (T) (the current defined by the onset of resistance). It is concluded that studying theI−V characteristics of amples in the context of theI−T phase diagram is a simple, useful tool for comparing samples. Work supported by the Office of Naval Research.     

Magnetoresistance relaxation and flux creep in high-Tc superconductors

The appearance of hysteretic voltages on ceramic high temperature superconductors during the measurement of the mixed state resistance is now well known., While these voltages can be attributed to a field dependence of the critical current density, the primary phonomenon is the V—H—i characteristic which is representative of a magnetic field induced, dissipative process in the granular material.

Two mechanisms may be responsible for these effects; one is the well known flux-flow or flux-creep phenomenon where the electrical energy is used in changing the vortex system while the other is a more conventional thermal loss as the intergranular contacts are driven normal by the applied field.

The observation of a corresponding time dependent residual voltage which is associated with flux trapped in the superconductor is examined in terms of the two models, both of which are found to predict the observed time decay and also justify the derivation of the flux-trapping energies from the decay law. The differences between the models appear in the prefactors to the logarithmic decay term.     

插层FeSe高温超导体的高压研究进展

通过对FeSe进行化学插层可以将其超导转变温度（T_c）从约8 K提高到40 K以上,实现高温超导电性.最近,我们对两种插层FeSe高温超导材料（Li_0.84Fe_0.16）OHFe_0.98Se和Li_0.36（NH₃）_yFe₂Se₂开展了高压调控研究,发现压力会首先抑制高温超导相（称为SC-I相）,然后在临界压力P_c以上诱导出第二个高温超导相（称为SC-Ⅱ相）,呈现出双拱形T-P超导相图.这两个体系的P_c分别约为5和2 GPa,两个体系SC-Ⅱ相的最高T_c分别可以达到约52和55 K,比相应SC-I相的初始T_c提高了10 K.对（Li_0.84Fe_0.16）OHFe_0.98Se的正常态电输运性质分析表明,SC-I和SC-Ⅱ相的正常态分别具有费米液体和非费米液体行为,意味着这两个超导相可能存在显著差异.此外,还发现这两个体系的SC-Ⅱ相的T_c与霍尔系数倒数1/R_H（∝载流子浓度n_e）具有很好的线性依赖关系.对（Li_0.84Fe_0.16）OHFe_0.98Se的高压X射线衍射测量排除了其在10 GPa以内发生结构相变的可能,因此P_c以上SC-Ⅱ相的出现和载流子浓度的增加很可能起源于压力导致的费米面重构.     

Symmetrical aspects of the cooperative pseudo Jahn-Teller effect in high-Tc superconductors

A microscopical model for the structural phase transitions in high-T_c La₂CuO₄-type superconductors is presented. This model is based on the cooperative pseudo Jahn-Teller effect. The most attention is paid to the symmetrical aspects of the approach. The developed theory predicts the possibility of the re-entrant structural phase transition from the orthorhombic phase to the tetragonal one in the doped superconductors. This prediction is in agreement with the experimental observation. The important consequences for the mechanism of superconductivity that followed from the structural phase transitions are under consideration too.     

Exact treatment of some phenomenological models of layered high-Tc superconductors

Existing treatments of phenomenological models of Bi and Tl based superconductors consisting of n layers of copper oxide are unsatisfactory because they ignore the important φ⁴ terms in the free energy. An exact treatment of these models is presented, and it is shown that the condition of minimum free energy automatically excludes (n − 1) of the n critical temperatures. The critical exponent β of the order parameter is found to be independent of n and equal to as usual in the Landau theory. The specific heat discontinuity Δc at the critical temperature is calculated as a function of n. A meaningful comparison with experimental measurements of Δc at the moment is hindered by the limited availability of experimental results. Reliable determinations of Δc of a given series of layered superconductors will enable a check of the validity of theoretical results.     

Random telegraph noise spectra in granular high-Tc films

Random telegraph noise voltages developing across current biased granular high-T_c films interact with the system of Abrikosov vortices pinned inside the grains through random Lorentz forces caused by the flow of random currents in the film. Viscous relaxation of vortices opposes random forces and leads to the attenuation of the low frequency part of the observed spectra of the voltage noise. The theoretical model has been created and discussed. The predictions of the model have been verified experimentally by observations of a peak in the random noise spectra recorded for granular YBaCuO thin film strips immersed in a weak magnetic field and biased with a dc current flow above the critical current. It has been found that the peak position and its changes with charging random telegraph switching rate are in a good agreement with the theoretical predictions.     

Correlation of Tc with structure in the density of states in the new high-Tc superconductors

The variation of T_c with hole concentration in the new copper oxide superconductors can readily be understood in terms of a peak in the density of states associated with the CuO₂ planes. The data are consistent with either simple phonon-mediated pairing or an indirect excitonic pairing. Effects due to interlayer coupling are considered. The sharp decrease of T_c near half-filling is probable due to strong spin fluctuations.     

Effective transfer for singlets formed by hole doping in the high-Tc superconductors

By comparing the exact results of an extended multiband Hubbard Hamiltonian for Cu₂O₇ and Cu₂O₈ clusters to those obtained from an effective single band Hamiltonian, we show that the low energy scale physics is very well described by a t-t′-J model which describes the motion of singlets in an antiferromagnetic background of spins. We obtain values for t, t′ and J for both hole and electron doping and show that these are different. We also study the dispersion relations and density of states within the quasi particle approximation both of which show rather interesting characteristics which could be relevant for high T_c superconductors.     

Simple method for investigating electromagnetic properties of high-Tc superconductors at two-millimeter wavelengths

A method for investigating the electromagnetic properties of high-T_c superconductors (HTS) in a 2-mm wavelength band has been proposed. The method is based on the use of a cylindrical tube open dielectric resonator (ODR) loaded with a HTS sample. The Josephson harmonic generation at microwave frequencies can be investigated by means of the method. A small cross section of the ODR makes it possible to investigate surface variations of HTS sample electromagnetic properties and their temperature dependencies.     

Infrared spectra of the high-Tc TlBaCaCuO superconductors

The TlBaCaCuO superconductors with zero resistance at temperatures in the range of 100–123 K have been sintered by solid state reactions. Electron microscopy and X-ray diffraction studies reveal that they are multiphased, containing both Tl₂Ba₂CaCu₂O₈ and Tl₂Ba₂Ca₂Cu₃O₁₀ superconducting phases. Infrared spectra have been measured at room temperature for the multiphase samples. The spectra display structures at 590 cm^-1 (P₁) and 530 cm^-1 (P₂) which seem characteristic to Cu-O stretching modes. The difference ΔP between P₁ and P₁ has been found to be related to the superconducting transition temperature. Smaller ΔP corresponds to a higher T_c. The electron-like excitation of0.1 eV and the phonon-like structure at 850 cm^-1 are also found to be important for high T_c.     

Double-step behavior of critical current vs. magnetic field in Y-, Bi- and Tl-based bulk high-Tc superconductors

A double step characteristic is observed at 76 K in the transport critical current as a function of magnetic field (10^-4 T to 10 T) in bulk sintered Y-, Bi- and Tl-based high-T_c superconducting materials. The low-field, step-like drop in the critical current density J_c commences at magnetic fields B between about 0.3 and 2 mT. This is followed by a plateau region of relatively constant critical current extending from about 30 to 300 mT, and then a second drop at fields between about 0.3 and 10 T. These features occur for all three superconductor materials and are interpreted respectively as a self-field/weak-link regime, a remnant percolation path regime and a flux-flow/upper-critical-field regime. The sharpness of the transition of the voltage-current (V-I characteristic, represented by the transition parameter n (i.e., V∝Iⁿ), has a similar double-step shape as a function of magnetic field directly corresponding to the features of the J_c(B) characteristic.     

Properties of a local pairing model of high-Tc superconductivity

A simulation technique is used to study the properties of the “hole modulated hopping” model introduced by Hirsch. The superconducting order parameter, energy gap and pair size have been determined for a range of particle densities and temperatures in the neighbourhood of the superconducting phase transition. Results are consistent with the interpretation of the superconducting transition to be Bose-like at low hole densities and BCS-like at high hole densities, with a crossover near the T_c maximum in the T_c versus hole density curve. This behaviour is related to the existence of small non-overlapping pairs at low hole densities and large strongly interpenetrating pairs at hole densities above the T_c maximum.     

Properties of high-Tc superconducting circular waveguides with Meissner boundary

The propagation properties of the TE-modes in a high-temperature superconducting circular waveguide using the Mei\ner boundary conditions on the wall are presented for the first time. The results show that now the normalized cutoff parameterk _c R, (whereR is the radius of the superconducting circular waveguide andk _c the cutoff wavenumber,) is dependent on the radius unlike conventional metallic circular waveguide whose normalized cutoff parameterk _c R is a constant for a given mode and the filled dielectrics. Instead of TE₁₁-mode now TE₀₁ mode becomes the dominant mode and the normal component of magnetic field for the dominant mode is not equal to zero on the wall. Other unique results of high-T _c superconducting circular waveguides are illustrated, too.supported by Deutscher Akademischer Austauschdienst (DAAD)     

On the photoelectron spectra of UO2

The Mg Kα and He(II) photoelectron spectra of polycrystalline UO₂ have been recorded. Particular care was taken to prepare a sample surface which was both representative of the bulk and as close to the ideal stoichiometry as possible. The origin of satellite structure in core-level spectra is discussed with the aid of a multi-configuration Dirac—Fock atomic calculation.     

Isolated vortex structure and the lower critical field for clean type II superconductors far from Tc

The structure of an isolated vortex line and the lower critical field have been calculated numerically from Eilenberger's version of the Gorkov theory. Results for clean superconductors with κ between $\text{1}\text{√2}$and 2 and temperatures down to 0.25 T_c are reported and compared with experiment.     

Temperature dependence of the far infrared reflectivity spectra of a KD2PO4

Complete data on far infrared reflectivity spectra of a KD₂PO₄ single crystal in the temperature range 7–300 K, from the millimeter wave up to 400 cm^–1, for all crystallographic orientations of the crystal, in the ferro- and paraelectric phase, are given for the first time. The temperature dependence of the phonon spectra is similar to that we have observed and published for KH₂PO₄.